Self‐Assembled Fluorescent Block Copolymer Micelles with Responsive Emission

Abstract Responsive fluorescent materials offer a high potential for sensing and (bio‐)imaging applications. To investigate new concepts for such materials and to broaden their applicability, the previously reported non‐fluorescent zinc(II) complex [Zn(L)] that shows coordination‐induced turn‐on emission was encapsulated into a family of non‐fluorescent polystyrene‐block‐poly(4‐vinylpyridine) (PS‐b‐P4VP) diblock copolymer micelles leading to brightly emissive materials. Coordination‐induced turn‐on emission upon incorporation and ligation of the [Zn(L)] in the P4VP core outperform parent [Zn(L)] in pyridine solution with respect to lifetimes, quantum yields, and temperature resistance. The quantum yield can be easily tuned by tailoring the selectivity of the employed solvent or solvent mixture and, thus, the tendency of the PS‐b‐P4VP diblock copolymers to self‐assemble into micelles. A medium‐dependent off–on sensor upon micelle formation could be established by suppression of non‐micelle‐borne emission background pertinent to chloroform through controlled acidification indicating an additional pH‐dependent process.


Experimental section
H2(L) and [Zn(L)] were synthesized as described in literature. [1] Please note that the complex is only sparingly soluble in neat toluene even at a lower concentration of 0.025 g/L. The polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) diblock copolymers (BCPs) were synthesized by sequential anionic polymerization of styrene and 4-vinylpyridine according to literature. [2] The nanocontainers are based on three different PS-b-P4VP-BCPs with varying composition but comparable overall molecular weights (S58V42 157 , S65V35 131 , and S85V15 154 : subscripts give the fraction of the respective block in wt%; superscript denotes the number average molecular weight in kg mol -1 ). [3] Toluene was of analytical grade and used without further purification.
Chloroform was extracted with aqueous saturated NaHCO3 solution and dried over CaCl2. CHCl3 (acidic) was prepared by extracting 50 mL dried CHCl3 three times with 15 mL 4 M hydrochloric acid.
[Zn(L)]@BCP. Diblock copolymer SxVy z (0.150 g) and [Zn(L)] (0.0015 g) were heated in 60 mL toluene for 2 h under reflux. After cooling, the solvent was removed in vacuo and the yellow solid was dried in vacuo.

Characterization
Transmission electron microscopy (TEM) measurements were performed on a CEM902 microscope from Carl Zeiss Microscopy (Oberkochen, Germany). Samples were dispersed in toluene (c = 0.67 g L -1 ) and the unfiltered solutions were dropped directly on a carbon coated TEM grid. The measurements were performed at an electron acceleration voltage of 80 kV. Micrographs were taken with an Orius 830 SC200W/DigitalMicrograph version 2.3 system from Gatan (Munich, Germany).
Measurements were also performed on a Zeiss/LEO EM922Omega (Carl Zeiss Microscopy, Oberkochen, Germany) at an acceleration voltage of 160kV. Micrographs were taken with a CCD UltraScan camera system (Gatan, Munich, Germany) and acquiring software Digital Micrograph version 1.9 (Gatan, Munich, Germany). The software "ImageJ" developed by Wayne Rasband was used for the particles size determinations. [4] The diameter of 150 particles was determined and averaged.

Dynamic light scattering (DLS) measurements were conducted on an AntonPaar
Litesizer 500 in fluorescence quartz glass cuvettes with a 1 cm light pathway from Hellma. The measurements were performed in backscattering mode and consisted of -S3-six consecutive runs. The samples were dispersed in toluene (c = 0.2 g L -1 ) and were not filtered. The experimental data were fitted with a cumulative fit. Measurements were also performed on an ALV DLS/SLS-SP 5022F compact goniometer system with an ALV 5000/E cross-correlator at a scattering angle of q = 90° and at T = 23 °C, using a HeNe laser (max. 35 mW, l = 632.8 nm) as the light source. The time-dependent scattering intensity was monitored with an APD (avalanche photodiode)-based pseudo cross correlation system. All samples were filled into NMR tubes (VWR, 5 mm outer diameter) for measurement. For each sample at least 3 measurements were performed. The data were evaluated using ALV Correlator software (version V.3.0.0.17 10/2002) and the implemented ALV regularized fit option (g2(t), CONTIN-analysis).

Absorption spectra were performed on a Cary 60 UV-Vis spectrometer from Agilent
Technologies. The samples were dispersed in toluene (c = 0.2 g L -1 ) and were not filtered. For steady-state photoluminescence (PL) a FP-8600 fluorescence spectrometer from JASCO was employed that is equipped with a 150 W Xe lamp as excitation source. Time-correlated single photon counting (TCSPC) measurements to determine emission lifetimes were performed on a FluoTime 300 spectrofluorometer from PicoQuant, using a 405 nm diode laser for excitation (Coherent COMPASS 405-50 CW), which was controlled by the PDL 820 PicoQuant laser driver. Quantum yields were determined at room temperature using a 78mm integrating sphere and a 300 W Xe lamp as excitation source. All measurements were conducted in quartz cells with a 1 cm lightpath from Hellma.

Correction of the optical data
Neat BCP in the respective solvent mixture was used as background for the [Zn(L)]@BCP samples in the absorbance measurements to subtract the contributions of the neat BCP micelles on the absorption behavior (see Figure S5A/C/D and S15A in the SI). Please note, that the emission data was corrected against the absorbance at the excitation energy.

Calculation of equivalents of vinylpyridine compared to [Zn(L)(H2O)(MeOH)]
The calculation is exemplary given for the S58V42 157 BCP.          In the molecularly dissolved form of the BCP, the BCP is solved in 30 mL chloroform. In the micelle form of the BCP, the poly(4-vinylpyridine) block forms the core of the micelles (diameter 43 nm). Please note, that the calculated values are based on the following assumptions: -All polymer chains form micelles -All pyridine substituents are available for ligation For this reason, the exact values are not determinable. However, the order of magnitude shows that the local pyridine concentration inside the micelles is strongly increased compared to the molecularly dissolved form.